Efficient, narrow-band, and stable electroluminescence from organoboron-nitrogen-carbonyl emitter

Organic light-emitting diodes (OLEDs) exploiting simple binary emissive layers (EMLs) blending only emitters and hosts have natural advantages in low-cost commercialization. However, previously reported OLEDs based on binary EMLs hardly simultaneously achieved desired comprehensive performances, e.g., high efficiency, low efficiency roll-off, narrow emission bands, and high operation stability. Here, we report a molecular-design strategy. Such a strategy leads to a fast reverse intersystem crossing rate in our designed emitter h-BNCO-1 of 1.79×105 s−1. An OLED exploiting a binary EML with h-BNCO-1 achieves ultrapure emission, a maximum external quantum efficiency of over 40% and a mild roll-off of 14% at 1000 cd·m−2. Moreover, h-BNCO-1 also exhibits promising operational stability in an alternative OLED exploiting a compact binary EML (the lifetime reaching 95% of the initial luminance at 1000 cd m−2 is ~ 137 h). Here, our work has thus provided a molecular-design strategy for OLEDs with promising comprehensive performance.

Organic light-emitting diodes (OLEDs) exploiting simple binary emissive layers (EMLs) blending only emitters and hosts have natural advantages in low-cost commercialization.However, previously reported OLEDs based on binary EMLs hardly simultaneously achieved desired comprehensive performances, e.g., high efficiency, low efficiency roll-off, narrow emission bands, and high operation stability.Here, we report a molecular-design strategy.Such a strategy leads to a fast reverse intersystem crossing rate in our designed emitter h-BNCO-1 of 1.79×10 5 s −1 .An OLED exploiting a binary EML with h-BNCO-1 achieves ultrapure emission, a maximum external quantum efficiency of over 40% and a mild roll-off of 14% at 1000 cd•m −2 .Moreover, h-BNCO-1 also exhibits promising operational stability in an alternative OLED exploiting a compact binary EML (the lifetime reaching 95% of the initial luminance at 1000 cd m −2 is ~137 h).Here, our work has thus provided a molecular-design strategy for OLEDs with promising comprehensive performance.
Organic light-emitting diode (OLED) technology has shown great potential for applications in next-generation displays and lighting 1,2 .Binary emissive layers (EMLs) composed of only hosts and emitters have natural advantages in low film-fabrication cost, thus making them the mainstream solution in current OLED production lines.However, it is difficult in previously reported OLEDs with binary EMLs to simultaneously show desired comprehensive performances to date, e.g., high color purity, high efficiency, low efficiency roll-off, and long operational lifetime.
To overcome the above long-standing bottleneck, one of the keys is to design novel OLED emitter molecules.Since the seminal work in 2012 3 , high-efficiency organic emitters with thermally activated delayed fluorescence (TADF) have attracted much attention from industry and academia, though the color purities of the first-generation TADF materials are not ideal 4,5 .In 2016, Hatakeyama et al. pioneered a rigid fused organoboron-nitrogen (BN) framework 6 .Their distinct BN-based frameworks induce a small energy gap (ΔE S1T1 ) between the first singlet (S 1 ) and triplet (T 1 ) excited states and thus TADF via the so-called multi-resonance (MR) effect, with substantially suppressed geometrical relaxations and thus a narrow emission bandwidth [7][8][9][10][11][12][13][14] .However, OLEDs based on such BN-based MR emitters showed serious efficiency roll-off and poor device operation stability.One of the main reasons is the slow reverse intersystem crossing (RISC) rate (k RISC ) from the triplet to singlet excited states, generally within 10 2 -10 5 s −1 in BN-based MR emitters 9,12,15 .The urgent task is thus to propose a judicious molecular-design strategy for MR emitters with a faster k RISC (e.g., >10 5 s −1 ), narrow emission bandwidths, and high photoluminescence (PL) quantum yields (Φ PL s).
In the framework of the Fermi golden rule, k RISC depends on the square of spin-orbit coupling (SOC) between T 1 and S 1 states and the negative exponent of ΔE S1T1 16 .A large SOC and small ΔE S1T1 are important to accelerating the RISC process.However, the conventional BN-based MR emitters can gain small ΔE S1T1 (e.g., <0.1 eV); their SOC values are commonly small or negligible, usually ~0.1 cm −1 12 .Recently, the heavy atoms [e.g., selenium (Se)] were incorporated into BN-based MR emitters to enhance SOC (e.g., up to ~2.8 cm −1 ) via the heavy-atom effect 17,18 , eventually leading to a fast k RISC (2.0 × 10 6 s −1 ).Although a promising external quantum efficiency (EQE) of ~36.8% in an OLED with a binary EML composed of a bipolar TADF host and Se-integrated MR emitter was obtained, its spectral full width at half maximum (FWHM) was as broad as 48 nm, which is much broader than that (ca.34 nm) of its parent compound BNCZ (for its chemical structure, see Fig. 1a) 17,19 .The broader FWHMs of the Se-integrated MR emitters are because the larger-size electronic cloud of the Se atom can be more easily deformed with molecular vibrations, thus increasing geometrical relaxation energies 20 .Moreover, OLEDs exploiting the Seintegrated MR emitters show short device lifetimes, e.g., LT 80 (the lifetime attenuating to 80% of the initial luminance) at an initial brightness of 1000 cd m −2 ~1 h 17 , which could be ascribed to high chemical activity of organic selenides.While the broader FWHM issue can be addressed by moving heavy atoms to a peripheral decoration rather than MR frameworks, the device's operational stability is still not satisfactory 21 .Therefore, the design and development of MR-TADF emitters composed of only Period-2 elements with a fast k RISC is critical.
In the present work, we thus rationally proposed a promising molecular-design strategy, i.e., hybridization of organoboron-nitrogen and carbonyl (named h-BNCO) fragments.Via such a strategy, we fused carbonyl groups into the conventional BNCZ framework and designed a proof-of-concept h-BNCO molecule, i.e., h-BNCO-1 (Fig. 1a).Compared with BNCZ, h-BNCO-1 not only reduces ΔE S1T1 and the T 1 -T 2 energy gap but also substantially increases the SOC between the S 1 and T 2 states, eventually accelerating its k RISC to 1.79 × 10 5 s −1 .Moreover, a small FWHM is maintained in h-BNCO-1.An OLED exploiting a binary EML with h-BNCO-1 doped in a bipolar TADF host yielding a high Φ PL of 99.6% ± 2% not only achieved ultrapure green electroluminescence (EL) with an FWHM of 39 nm, corresponding to CIE coordinates of (0.24, 0.71) but also showed an impressive maximum EQE of 40.1% with a mild efficiency roll-off of 14% at 1000 cd m −2 .Moreover, h-BNCO-1 also exhibited promising operational stability in an alternative OLED exploiting a compact binary EML, with device lifetime LT 95 at an initial brightness of 1000 cd m −2 reaching 137 h.Such device performances and stability thus represent the state-of-the-art comprehensive OLED performances.Our work has thus provided a promising h-BNCO molecular-design strategy for OLEDs exploiting binary EMLs with high efficiency, high color purity, low-efficiency roll-off, and promising operational stability.

Theoretical calculations and h-BNCO molecular-design strategy
To first provide a fundamental understanding of the electronic structure of our designed h-BNCO-1 and the conventional BNCZ, quantumchemistry calculations were carried out.Previous theoretical investigations have demonstrated that the electron correlation effect is important in MR molecules, and popular DFT approaches cannot provide an exact description of the energies and characteristics of their excited states [22][23][24] .Here, the high-level Coupled-Cluster quantumchemistry approach was exploited to examine the electronic-structure properties of the excited states (for details, see the Computational details section).
As shown in Fig. 1, the conventional BNCZ with a small distorted angle (~8.4°) between carbazole (CZ) and DABNA fragments has a ΔE S1T1 of 0.2 eV and a ΔE T2T1 of 0.26 eV.The SOC between the S 1 and T 1 /T 2 states is small, ~0.07/0.29 cm −1 .The combination of small SOC values, large ΔE S1T1 , and large ΔE T2T1 thus leads to a slow k RISC (~1.57× 10 4 s −1 ).Compared with BNCZ, h-BNCO-1 shows a much more distorted geometry, with a dihedral angle of ~25°between the acridone and DABNA groups.Due to the electron-withdrawing effect of the carbonyl groups and the electronic delocalization on the carbonyl groups, the S 1 -and T 1 -state energies both decrease, with a reduced ΔE S1T1 of 0.12 eV.Moreover, ΔE T2T1 decreases to 0.18 eV, which is important to accelerating the RISC process.The decrease in ΔE T2T1 (i.e., also the energy splitting between the triplet states of the two same fragments in h-BNCO-1) could be induced by its highly distorted geometry since the electronic communication between the two same fragments is blocked.The SOC(S 1 -T 1 ) is negligible, while the SOC(S 1 -T 2 ) is very large, ca.1.26 cm −1 .This difference in their SOC values is mainly due to the difference in the excited-state characteristics since the total angular momentum for the electrons must be conserved in any spin-flip event.The T 1 and S 1 states both mainly show a similar ππ* excitation character, although on the carbonyl groups the T 1 state shows a mixing of ππ* and nπ* (see the region circled by the blue-dashed line in Fig. 1d and Supplementary Fig. 1).In contrast to the T 1 state, the ππ* excitation character of the T 2 state on the BN main body shows an apparent difference with that of the S 1 state, as highlighted by the black-dashed line in Fig. 1d.Moreover, the T 2 state on the carbonyls shows only nπ* excitation (see the region circled by the green-dashed line in Fig. 1d), which is different from the ππ* excitation character of the S 1 state on the groups.Therefore, such a combination of small ΔE S1T1 , small ΔE T2T1 , and large SOC(S 1 -T 2 ) gives rise to a fast Boltzmann-averaged k RISC value of ~1.25 × 10 5 s −1 .
To further demonstrate the advantage of our proposed h-BNCO molecular-design strategy, we replaced the two carbonyls of h-BNCO-1 with the electron-withdrawing phenyl-boron groups and thus designed a proof-of-concept BN-MR molecule (named BNBNB) without the carbonyls.The results of our theoretical calculations on BNBNB are also shown in Fig. 1 and the corresponding data are summarized in Supplementary Table 1.Clearly, compared with BNCZ, the introduction of phenyl-boron groups also leads to a smaller ΔE S1T1 (0.08 eV) and ΔE T2T1 (0.18 eV), similar to h-BNCO-1.However, its SOC value (0.69 cm −1 ) between the S 1 and T 2 states is smaller than that in h-BNCO-1 due to the absence of the nπ* excitation in the T 2 state.Eventually, the Boltzmann-averaged k RISC value (~1.70 × 10 3 s −1 ) in BNBNB is two orders of magnitude slower than that in h-BNCO-1.In addition, a few examples of nitrogen-carbonyl-conjugated frameworks were reported to show MR-TADF properties.However, their device performances were less ideal, showing broader spectral bands and poor efficiencies with serious efficiency roll-offs [25][26][27][28][29][30][31][32][33][34] .In short, our calculation results have thus demonstrated that the proposed h-BNCO strategy not only reduces ΔE S1T1 and ΔE T2T1 but also increases the SOC values, eventually substantially accelerating the RISC process.

Photophysics
UV-vis absorption and PL spectra of BNCZ and h-BNCO-1 were recorded in toluene (1.0 × 10 −5 M) at room temperature.As shown in Fig. 2a and Supplementary Fig. 2a, BNCZ, and h-BNCO-1 both display strong and sharp absorption bands at 458 and 489 nm, respectively.The PL spectrum of BNCZ shows a maximum at 477 nm with an FWHM of 0.13 eV (23 nm); in contrast, that of h-BNCO-1 shows an evident bathochromic shift to 516 nm due to the incorporation of carbonyl groups, while importantly, its FWHM is well maintained at 0.13 eV (corresponding to 28 nm).Furthermore, in more solvents with various polarities, such PL trends of both emitters basically remained (see Supplementary Fig. 2b, c).
To further determine the key energy levels of both compounds, we measured their fluorescence and phosphorescence spectra in frozen toluene at 77 K (shown in Supplementary Fig. 3 and the data are summarized in Table 1).Based on the fluorescence and phosphorescence maxima of BNCZ and h-BNCO-1, the energies of their S 1 states are estimated to be 2.57 and 2.39 eV, and those of their T 1 states are estimated to be 2.46 and 2.36 eV, respectively.Their ΔE S1T1 values were further computed to be 0.11 and 0.03 eV, respectively.This indicates that the introduction of carbonyl groups substantially reduces ΔE S1T1 , thus benefitting the RISC process, which is consistent with our theoretical calculation results.
We then prepared a film of 1 wt% h-BNCO-1 doped in a non-TADF material 3,3-di(9H-carbazol-9-yl)biphenyl (mCBP).The Φ PL of the h-BNCO-1-based film was evaluated to be 75.0%± 2%. Figure 2b and Supplementary Fig. 4 depict its transient PL decays at room temperature.An obvious delayed component is detected due to the TADF process.We further evaluated photophysical dynamics based on these data (summarized in Fig. 2b).Importantly, the k RISC of h-BNCO-1 reaches an impressive value of 1.79 × 10 5 s −1 , which is much faster than that of BNCZ in the same condition (1.24 × 10 4 s −1 ) 35 .The substantially accelerated RISC can be well explained by our theoretical estimation results.The higher-lying T 2 state in h-BNCO-1 shows a larger SOC with the S 1 state due to carbonyl incorporation, which contributes to the overall RISC process.For OLEDs, the substantially enhanced RISC process can not only enhance triplet utilization but also reduce tripletpolaron/triplet annihilation at high exciton densities, which can give rise to high peak efficiency, suppressed efficiency roll-off at high current densities as well as operational stability.
The device performances are summarized in Table 2 and shown in Supplementary Fig. 6.As shown in Fig. 3b, the BNCZ-based device shows a sky-blue EL spectrum with a peak at 484 nm and a FWHM of 34 nm (178 meV) and the h-BNCO-1-based device shows a green EL spectrum with a peak at 528 nm, an FWHM of 39 nm (173 meV) and a nearly Lambertian emission pattern (Supplementary Fig. 7a).With an appropriate EL peak as well as a narrow spectral bandwidth, the h-BNCO-1-based device finally operates at high-quality green CIE coordinates of (0.24, 0.71), which is very close to the CIE coordinates of (0.21, 0.71) in the green standard defined by the National Television System Committee (NTSC) (see Supplementary Fig. 7b).Moreover, the device based on h-BNCO-1 exhibits a maximum EQE of 40.1%, which is evidently superior to the BNCZ-based device (12.1%) and among the highest EQEs in OLEDs.Importantly, at an initial brightness of 1000 cd m −2 , the h-BNCO-1-based device still maintains a decent EQE of 34.6%, corresponding to a relative roll-off of 14%.Such efficiency rolloff behavior in the h-BNCO-based device is superior to not only the BNCZ-based device but also most of the ever-reported OLEDs exploiting binary EMLs with MR-emitters (see Supplementary Table 2).These device results further demonstrate the importance of h-BNCO in accelerating the RISC process and relieving exciton quenching at high current densities (see Supplementary Fig. 7c).The h-BNCO-1-based device simultaneously achieves high EQE, low-efficiency roll-off, and high color purity, which apparently surpasses the reported OLEDs.

Discussion
In conclusion, we proposed a promising molecular-design strategyhybridization of organoboron-nitrogen and carbonyl (h-BNCO)-for MR emitters to achieve high color purity, high efficiencies, lowefficiency roll-off, and promising operational stability in OLED devices, which goes beyond the conventional ones.Compared to the control compound BNCZ, our designed h-BNCO-1 shows a similar narrow fluorescence-emission bandwidth of 0.13 eV and a higher Φ PL (99.6% ± 2%).Importantly, h-BNCO-1 achieves a fast k RISC of 1.79 × 10 5 s −1 .This is mainly because the introduction of two carbonyl groups in h-BNCO-1 induces a smaller ΔE S1T1 , a smaller ΔE T1T2 , and a larger T 2 -S 1 SOC value, thus facilitating the T 2 -to-S 1 upconversion channel, which makes a large contribution to the overall RISC process.Ultimately, the optimized OLED based on binary EML with h-BNCO-1 achieves an ultrapure green EL with a spectral FWHM of 39 nm and CIE coordinates of (0.24, 0.71).Meanwhile, thanks to the enhanced RISC process, the device EQE not only achieves a maximum value of 40.1%  but also shows a much smoother efficiency roll-off compared to most of the device behaviors of the reported MR-TADF emitters.Moreover, h-BNCO-1 has demonstrated promising operational stability in OLEDs.
In a device with a binary EML, a lifetime LT 95 , as long as 137 h, is determined at an initial brightness of 1000 cd m −2 .
In the aspect of OLED material chemistry, our work provided a promising chemical paradigm of h-BNCO for designing more MR-TADF molecules; in the aspect of OLED devices, the impressive performance and stability obtained via a simple binary-EML structure with h-BNCO-1 demonstrated the potential for low-cost commercialization based on h-BNCO type MR-TADF molecules.While the EL color purity is slightly inferior to that of the previously reported purest green MR-TADF molecule 37 , we believe it can be addressed by appropriately locking the phenyl-rings with the central phenyl-ring in h-BNCO-1 molecule (e.g., using spiro-carbon formation) to further suppress the excited-state geometrical distortion and vibration modes.Further molecular design and optimization are ongoing based on this concept.

Molecular synthesis and characterization
Current MR compounds with boron-fused polycyclic aromatics are synthesized via one-shot/one-pot borylation reactions based on an intramolecular tandem bora-Friedel-Crafts reaction, in which lithiumhalogen exchange by using organic Li reagents (RLi) is generally carried out at the first step, and then a borylation reaction occurs via suitable boron trihalides.However, for a carbonyl-containing reaction substrate, on the one hand, RLi, as a highly active nucleophile, can easily react with carbonyl groups [38][39][40][41][42][43] ; on the other hand, carbonyl groups substantially reduce the activity of aromatics for the Friedel-Crafts reaction.Therefore, h-BNCO frameworks have not been reported to the best of our knowledge.Herein, the synthesis route of our target h-BNCO framework h-BNCO-1 is displayed in Supplementary Fig. 11.Precursor 3 was first synthesized via the Buchwald-Hartwig reaction.Then, the preliminary B/N-based MR compound 4 containing a methylene group was synthesized by using 3 as the reaction substrate through a one-pot lithiation-borylation-annulation reaction.Finally, by using 4 as the intermediate, the target molecule h-BNCO-1 was obtained by further oxidizing the methylene for the carbonyl group.The detailed synthetic routes and characterization data, including NMR and high-resolution mass spectral (HRMS) analysis of all newly reported molecules, are shown in Supplementary Figs.12-20 and the supplementary information.Moreover, the molecular geometry and packing mode of h-BNCO-1 in a single crystal sample were analyzed via X-ray diffraction (XRD) (see Supplementary Fig. 21, Supplementary Data 1, and Supplementary Tables 4 and 5).

Material
All reagents were purchased from commercial sources and used without further purification. 1H NMR and 13 C NMR spectra were recorded on a Bruker 600/151 MHz spectrometer in deuterium reagent at room temperature.MALDI-TOF mass data were recorded on a Bruker ultrafleXtreme instrument.

Computational details
To examine the equilibrium geometries of the ground and excited electronic states, the range-separated hybrid density functional ωB97XD with the nonempirical tuned ω value and the 6-31 G(d,p) basis set was employed here.All these quantum-chemistry calculations were performed using the Gaussian 16 software package 44 .Based on the optimized ground-state geometries, the high-level wavefunctionbased STEOM-DLPNO-CCSD method with the basis set def2-TZVP implemented in ORCA 4.2.1 software 45 was exploited to examine the excited-state energies and difference-density plots of the studied molecules.
With the calculated electronic structure parameters in Supplementary Table 1 as the input, the rate constant (k T1→S1 /k T2→S1 ) of the T 1 → S 1 / T 2 → S 1 process was evaluated via Marcus theory based on timedependent perturbation theory: and the Boltzmann-averaged k RISC was evaluated via: where k B is the Boltzmann constant; T is the temperature; and λ T 1=2 ÀS 1 denotes the reorganization energy related to the transition from T 1 /T 2 to S 1 .

Thermal stability
The thermal stability of BNCZ and h-BNCO-1 was evaluated via thermogravimetric analysis (TGA).As shown in Supplementary Fig. 22, the decomposition temperatures (T d , corresponding to 5% weight loss) of BNCZ and h-BNCO-1 are estimated to be 359 °C and 383 °C, respectively.The even higher T d of h-BNCO-1 indicates that introducing carbonyl groups further improves thermal stability, which is essential for the thermal evaporation process.

Electrochemical measurement
Cyclic voltammetry (CV) measurements were carried out for BNCZ and h-BNCO-1 (Supplementary Fig. 23).Based on the onsets of the oxidation and reduction curves, the HOMO energy levels are estimated to be −5.61 and −5.83 eV, and the LUMO energy levels are estimated to be −2.94 and −3.29 eV for BNCZ and h-BNCO-1, respectively.With carbonyl incorporation in the polycyclic aromatics, LUMO energy levels are more substantially deepened than HOMOs.The energy gaps (E g ) of h-BNCO-1 (2.54 eV) are slightly narrower than that of BNCZ (2.67 eV), which would lead to a redshift emission.

Photophysical measurements
UV-vis absorption spectra were recorded on a Hitachi U-3900 spectrophotometer.PL spectra were recorded on a Hitachi F-4600 fluorescence spectrophotometer.Transient fluorescence decays were measured with a time-resolved photoluminescence spectrometer (Edinburg FLS1000).The absolute PLQY was recorded on a Hamamatsu Quantaurus-QY quantum yield spectrometer (C13534-11).

Molecular orientation measurement
We evaluated the molecular orientation of 1 wt% emitter doped in a DMIC-TRZ film via angle-dependent PL measurements.As shown in Supplementary Fig. 24, a much higher horizontal molecular orientation ratio (Θ // ) of 98% is detected for h-BNCO-1 compared with 70% for BNCZ, which implies that devices based on h-BNCO-1 should exhibit higher light outcoupling efficiency than those based on BNCZ.

Device characterization
The electrical characteristics of the devices were measured with a Keithley 2400 source meter.The EL spectra and luminance of the devices were obtained on a PR655 spectrometer.Note that we also prepared a device with 1 wt% BNCZ doped in 9-(2-(9-phenyl-9H-carbazol-3-yl)phenyl)-9H-3,9′-bicarbazole (PhCzBCz) film as an alternative EML to achieve better device efficiency (see Supplementary Fig. 25).

Device operational lifetime measurement
The operational lifetime, driving voltage, luminance, and electroluminescent spectra of the encapsulated OLED devices were recorded and measured using a luminance meter (SR-3AR, TOPCON, Japan) under a constant current density with an initial luminance of 1000 cd m −2 . Open

bFig. 3 |
Fig. 3 | Optimized EL performances of BNCZ-and h-BNCO-1-based OLEDs.a Device structures with ionization potential and electron affinity (in eV) for each material (HTL hole-transporting layer, EML emitting layer, ETL electrontransporting layer), and the relevant chemical structures of the materials used in the EMLs.b Normalized EL spectra.c EQE−luminance characteristics and efficiency roll-offs of the OLEDs, in which the black dashed line marks the luminance of 1000 cd m −2 , and the green and blue marks represent the device roll-off results at 1000 cd m −2 of the devices based on h-BNCO-1 and BNCZ, respectively.

Fig. 4 |
Fig. 4 | Operational stability of the h-BNCO-1-based OLED. a The relative luminance versus operating time characteristics at an initial luminance of 1000 cd m −2 .b A summary of the operational lifetime (LT 95 at an initial brightness of 1000 cd m −2 )-wavelength peak-CIE-y coordinate of the reported MR-OLEDs based on binary-EMLs.

Table 1 |
Photophysical properties of BNCZ and h-BNCO-1 a Peak wavelengths of absorption and fluorescence spectra in toluene.b Full-widths at half-maxima.c Lowest excited singlet (S 1 ) and triplet (T 1 ) energy levels are determined from the peak values of fluorescence and phosphorescence spectra, respectively, in dilute toluene at 77 K. d ΔES1T1 = S1 -T1.

Table 2 |
EL performances of the OLEDs using DMIC-TRZ as the host a Peak wavelength of the EL spectrum.
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